Various sensors have been developed for measuring the silicon content of molten metal in situ, i.e. without sampling of the metal. One such device is described in U.S. Pat. Nos. 4,657,641 and 4,708,783. The sensor includes a solid electrolyte capable of conducting oxygen ions, a reference electrode in contact with one surface of the solid electrolyte for providing a constant oxygen potential at a particular temperature of measurement, and an auxiliary electrolyte comprising SiO.sub.2 disposed in the immediate vicinity of the other surface of the solid electrolyte. The auxiliary electrolyte may consist of essentially pure silica. However, pure silica is said to be "not fully satisfactory, since it tends to be softened in the molten pig iron so that its surface configuration may be deformed owing to flows of the molten pig iron". The patents state preferably the auxiliary electrode is made of a compound, solution or mixture of SiO.sub.2 with metallic oxides which are more stable than SiO.sub.2 in the molten metal, such as Group IIA (alkaline earth), Group IA (alkali), Group IIIB and IVB of the Periodic Table. The auxiliary electrode may be in contact with the solid electrode or located in the immediate vicinity of the same. A porous auxiliary electrode is disclosed in FIGS. 7 and 8 of the patents. An auxiliary electrode consisting essentially of a two-phase mixture of ZrO.sub.2 and ZrSiO.sub.4 is most preferred. This auxiliary electrode is prepared by mixing particulate ZrSiO.sub.4 and a paste of ZrO.sub.2, coating the outer surface of the solid electrolyte with the paste mixture, and calcining the coating at a temperature of about 1300 to about 1500.degree. C. The preferred form of the device is also described in a paper entitled "Rapid determination of silicon activities in hot metal by means of solid state electrochemical sensors equipped with an auxiliary electrode" by M. Iwase published in Scand. J. Metallurgy 17, (1988), pages 50-56. Variations of the aforementioned sensor are also disclosed in a paper entitled "Development of electrochemical silicon sensors for iron and steel melts" by K. Raiber, S. W. Tu and D. Janke, published in Steel Research 1990, pages 430-437. The latter paper discloses the use of one of the sensors for the measurement of chromium activities in Fe--O--Cr and Ni--O--Cr melts. A similar sensor using a multi-oxide auxiliary electrode comprising a mixture of ZrO+ZrSiO.sub.4 +Na.sub.2 Si.sub.2 ZrO.sub.7 is disclosed in a paper entitled "Laboratory and In-Plant Tests of a Solid-State Silicon Sensor Incorporating a Mixture of ZrO.sub.2 +ZrSiO.sub.4 +Na.sub.2 Si.sub.2 ZrO.sub.7 as an Auxiliary Electrode for Rapid Determination of Silicon Levels in Blast Furnace Hot Metal" by K. Gomyo, I Sakaguchi, Y. Shin-ya, and M. Iwase, published in Transactions of the ISS July, 1991, pages 71-78.
Other types of sensors are described below. One other sensor is described in a paper entitled "Tri-phasic zirconia electrolyte for the in-situ determination of silicon activities in hot metal" by M. Iwase, H. Abe and H. Iritani published in Steel Research 59 (1988) No. 10, pages 433-437. The sensor consists of an electrochemical cell with a tri-phasic zirconia electrolyte of cubic ZrO.sub.2 -MgO solid solution and monoclinic ZrO.sub.2 and 2 MgO.SiO.sub.2 and a reference electrode of Mo+MoO.sub.2. The triphasic electrolyte is fabricated by mixing magnesia-stabilized zirconia and forsterite. This sensor is also described in two other papers: one entitled "Three-Phase Zirconia Sensor for Rapid Determination of Silicon Levels in Hot Metal" by K. Gomyo, I. Sakaguchi, Y. Shin-ya, and M. Iwase published in Transactions of the ISS March 1993, Pages 87-95 and another entitled "Solid state sensor for silicon in molten metals by zirconia-based electrolyte" by K. Gomyo, I. Sakaguchi, Y. Sin-ya, V. Lakshmanan, A. McLean and M. Iwase published in Solid State Ionics 1994 70/71, pages 551-554.
A sensor with a molten silicate electrolyte is described in a paper by K. Ichibara, D. Janke and H. J. Engell published in Steel Research 57 (1986) No. 4, Pages 166-171. Another sensor using a molten silicate electrolyte is described in a paper by F. Buiarelli and P. Granati entitled "New electrochemical probe for silicon determination in hot metal " published in Steel Research No. 2 1990, pages 60-63.
A sensor having a molten metal retention chamber is described in Japanese published application No. JP 63191056. A probe in which an ion-conductive silicate electrolyte is used is described in Japanese published application nos. JP 60113145, JP 60085361 and JP59073763.
Finally a sensor utilizing a mullite electrolyte and Cr--Cr.sub.2 O.sub.3 reference electrode is described in a paper by R. Inoue and H. Suito published in Transactions of the ISS April 1995, pages 51-57.
Construction of the prior art sensors is fairly complicated and difficult. Also such sensors are not completely reliable or accurate and are somewhat costly.